Throttle discovery concurrency when rate-limit remaining gets low

[JSONbored]

Problem: The fanout already records GitHub rate-limit remaining/reset for display purposes, but nothing actually slows its own concurrent fetching down in response — a discover run can burn through the remaining budget at full concurrency right up until it gets rate-limited.

Area: AMS / Performance

Proposal: Make the fanout's concurrency dynamically back off as recorded rate-limit remaining drops below a configurable threshold.

Deliverables:

• Concurrency that scales down automatically as rate-limit remaining approaches zero.

Acceptance criteria:

• A simulated low-remaining-budget scenario shows fanout concurrency dropping rather than continuing at full speed into a 403.

[linear-code]

JSO-3504